This invention relates to a method of making polybenzoxazole (PBO) membranes from self-cross-linkable aromatic polyimide polymer comprising both hydroxyl functional groups and carboxylic acid functional groups and the use of these membranes in separations of gas mixtures and liquid mixtures.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used commercially for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. It has been found that polymer membrane performance can deteriorate quickly. A primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation can be prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes. Although these pretreatment systems can effectively perform this function, the cost is quite significant. In some projects, the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the size of the pretreatment system or even total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. Another factor is that, in recent years, more and more membrane systems have been installed in large offshore natural gas upgrading projects. The footprint is a big constraint for offshore projects. The footprint of the pretreatment system is very high at more than 10 to 50% of the footprint of the entire membrane system. Therefore, removal of the pretreatment system from the membrane system has great economic impact, especially to offshore projects.
Aromatic polybenzoxazoles (PBOs), polybenzthiazoles (PBTs), and polybenzimidazoles (PBIs) are thermally stable ladder-like glassy polymers with flat, stiff, rigid-rod phenylene-heterocyclic ring units. The stiff, rigid ring units in such polymers pack efficiently, leaving very small penetrant-accessible free volume elements that are desirable to provide polymer membranes with both high permeability and high selectivity. These aromatic PBO, PBT, and PBI polymers, however, have poor solubility in common organic solvents, preventing them from being used for making polymer membranes by the most practical solvent casting method.
Thermal conversion of soluble aromatic polyimides containing pendent functional groups ortho to the heterocyclic imide nitrogen in the polymer backbone to aromatic polybenzoxazoles (PBOs) or polybenzthiazoles (PBTs) has been found to provide an alternative method for creating PBO or PBT polymer membranes that are difficult or impossible to obtain directly from PBO or PBT polymers by solvent casting (Tullos et al, MACROMOLECULES, 32, 3598 (1999)). A recent publication in the journal SCIENCE reported high permeability polybenzoxazole polymer membranes in dense film geometry for gas separations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). These polybenzoxazole membranes are prepared from high temperature thermal rearrangement of hydroxy-containing polyimide polymer membranes containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability (>100 Barrer) which is at least 10 times better than conventional polymer membranes.
Poly(o-hydroxy amide) polymers comprising pendent phenolic hydroxyl groups ortho to the amide nitrogen in the polymer backbone have also been used for making PBO membranes for separation applications (US 2010/0133188 A1).
One of the components to be separated by a membrane must have a sufficiently high permeance at preferred conditions or extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Permeance, measured in Gas Permeation Units (GPU, 1 GPU=7.5×10−9 m3 (STP)/m2 s (kPa)), is the pressure normalized flux and is equal to permeability divided by the skin layer thickness of the membrane. Commercially available polymer membranes, such as cellulose acetate and polysulfone membranes, have an asymmetric structure with a thin dense selective layer of less than 1 μm. The thin selective layer provides the membrane high permeance representing high productivity. Therefore, it is highly desirable to prepare asymmetric PBO membranes with high permeance for separation applications. One such type of asymmetric hollow fiber PBO membrane has been recently disclosed by Park et al. (US 2009/0297850 A1) and Visser et al. (Abstract on “Development of asymmetric hollow fiber membranes with tunable gas separation properties” at NAMS 2009 conference, Jun. 20-24, 2009, Charleston, S.C., USA). The asymmetric hollow fiber PBO membranes disclosed by Park et al. and Visser et al. were obtained from o-hydroxyl substituted polyimide asymmetric hollow fiber membranes via thermal rearrangement. However, Visser et al. found that the high temperature thermally rearranged asymmetric hollow fiber PBO membranes had low gas permeances (equivalent to a dense selective layer thickness of >5 μm). The low gas permeance is because the fiber shrank and the porous substructure collapsed during thermal rearrangement at temperatures higher than 300° C. Therefore, much more research is still required to reduce the excessive densification of the porous membrane substructure of asymmetric o-hydroxyl substituted polyimide membranes during thermal rearrangement at elevated temperature to make asymmetric PBO membranes.
Park et al. also disclosed asymmetric hollow fiber PBO membranes obtained from o-hydroxyl substituted polyamic acid asymmetric hollow fiber membranes via thermal rearrangement (WO 2009142433 and US 2009/0282982 A1).
The present invention provides a method of making polybenzoxazole (PBO) membranes from self-cross-linkable aromatic polyimide polymer comprising both hydroxyl functional groups and carboxylic acid functional groups and methods of using these membranes.